1. Field of the Invention
The present invention relates to a modified Y-zeolite, a process for preparing the modified Y-zeolite, and to an FCC catalyst containing the modified Y-zeolite.
2. Description of the Related Art
Hydrocarbon feeds can be catalytically converted to lower boiling products by contacting them with fluidised catalyst particles under appropriate conditions in a reaction zone. The catalyst used in this process is a so-called fluidised catalytic cracking (FCC) catalyst which comprises zeolite particles in a matrix. The zeolites which have been used in the preparation of FCC catalysts include synthetic faujasites or Y-zeolites and modifications thereof, such as ultrastable Y-zeolites (USY) and calcined rare-earth-exchanged Y-zeolites (CREY). In the cracking process, the catalyst particles are deactivated by the precipitation of coke, which is formed as a by-product of the cracking process, on the catalyst particles. The (partially) deactivated catalyst particles are removed from the reaction zone, freed of volatile components in a stripping zone, subsequently passed to a regeneration zone, and, following their regeneration by combustion of the coke with an oxygen-containing gas, fed back to the reaction zone.
In recent times, much attention has been paid to optimizing the properties of the Y-zeolites to be used in FCC catalysts, in order to obtain FCC catalysts which produce less coke and gas and are more selective to gasoline and other desirable products. One of the modifications which has been proposed is the incorporation of phosphorus into the catalyst composition. European patent application No. 0 252 761 describes a cracking catalyst comprising an ultrastable Y-zeolite which has been treated with a phosphorus-containing compound to decrease the coke and gas make of the catalyst.
U.S. Pat. No. 4,970,183 describes a cracking catalyst comprising a phosphorus-containing Y-zeolite which has been prepared by contacting the Y-zeolite with a phosphorus-containing compound and subsequently heating the Y-zeolite at a temperature of 400.degree.-800.degree. C. in a steam atmosphere. Again, this measure has been taken to decrease the coke and gas make of the catalyst.
Another measure which has been taken to improve the properties of the catalyst is the effecting of an aluminum exchange.
U.S. Pat. No. 4,357,265 describes a process for modifying Y-zeolites in which an NaY-zeolite is subjected to an ammonium exchange, a calcination step, and an aluminum exchange. The FCC catalyst comprising the modified Y-zeolite produces gasoline with an improved octane number compared to a catalyst comprising a Y-zeolite which has not been subjected to an aluminum exchange.
U.S. Pat. No. 3,455,842 describes a cracking catalyst comprising a Y-zeolite which has been subjected to an aluminum-exchange step followed by a steam calcination step. This catalyst has an improved selectivity to gasoline compared to catalysts which do not comprise aluminum-exchanged Y-zeolite.
U.S. Pat. No. 4,224,188 describes a cracking catalyst comprising a Y-zeolite which has been prepared from an NaY-zeolite by a combination of ammonium-exchange and aluminum-exchange, which steps may be carried out in various order. The activity of the FCC catalysts comprising the various zeolites is compared.
However, even though the above-mentioned patents describe FCC catalysts with improved properties compared to catalysts comprising unmodified Y-zeolite, there is still need for FCC catalysts which have an improved selectivity to gasoline and LCO, and a lower coke and gas make. An additional problem which is encountered in the use of the zeolites described in the above-mentioned references is that both the aluminum-exchange step and the phosphorus-treatment tend to lower the =C.sub.3 /C.sub.3 ratio. Since C.sub.3 olefins are valuable products, there is need for an FCC catalyst which will show improved gasoline and LCO selectivity, decreased coke make, with the selectivity to C.sub.3 olefins being maintained or even improved.